A pretreatment coating is often applied to metal substrates, especially metal substrates that contain iron such as steel, prior to the application of a protective or decorative coating. The pretreatment coating minimizes the amount of corrosion to the metal substrate. In addition, the pretreatment coating can affect the adhesion of subsequently applied decorative coatings such as paints and clear coats. Many of the present pretreatment coating compositions are based on metal phosphates, and/or rely on a chrome-containing rinse. The metal phosphates and chrome rinse solutions produce waste streams that are detrimental to the environment. As a result, there is the ever-increasing cost associated with their disposal. There is an interest to develop pretreatment coating compositions and methods of applying such compositions without producing metal phosphate and chrome waste solutions. It is also preferred, that these pretreatment coating compositions be effective in minimizing corrosion and enhancing decorative coating adhesion on a variety of metal substrates because many objects of commercial interest contain more than one type of metal substrate. For example, the automobile industry often relies on metal components that contain more than one type of metal substrate. The use of a coating composition effective for more than one metal substrate would provide a more streamlined manufacturing process.
The coating compositions of the present invention are called pretreatment coatings because they are typically applied after the substrate has been cleaned and before the various primer and decorative coatings have been applied. In the automotive industry, coatings often comprise the following layers in order from the substrate out: a pretreatment coating for corrosion resistance, an electrodeposited electrocoat, then a primer layer, a base coat paint, and then a top clear coat. In the present application, all coatings after the pretreatment coating are considered as paints unless otherwise noted. One known pretreatment coating is Bonderite® 958 available from Henkel Adhesive Technologies. The Bonderite® 958 provides a zinc-phosphate based conversion coating composition that includes zinc, nickel, manganese and phosphate. Currently, Bonderite® 958 is a standard conversion coating used extensively in the automotive industry.
In attempts to move away from conversion coatings that include heavy metals, which as used herein will be understood by those in the conversion coating arts to mean zinc, nickel, cobalt, manganese, and chromium, or that produce phosphate waste streams, a new class of environmentally friendly conversion coating compositions has been created. The new class of coatings generally comprises a zirconium-based conversion coating deposited on a metal substrate by contact with a working bath containing dissolved zirconium in the coating compositions. These conversion coating compositions, which are based on a zirconium coating technology, typically have no phosphates and no nickel or manganese. Zirconium-based coatings are finding increasing use in the automotive industry as a pretreatment coating.
Manufacturing plants' metal coating assembly lines are part of an overall process that is highly coordinated and carefully timed. Metal workpieces are cut to size, formed, cleaned, coated with a pretreatment coating, and then coated with several over layers. Several different types of metal may pass separately through parts of the process to be joined to each other in one step and then proceed through the remaining process steps as an assembly of dissimilar metals. These processes are carried out on hundreds of pieces per hour and the system requires precise movement of a metal workpiece through the process. From time to time, the processing line may be halted, sometimes unexpectedly due to a problem in one of the processes in the assembly line. When line stoppage occurs, workpieces are held in the various stages of the line for far longer than is desirable. When a workpiece is held in a pretreatment bath too long it is often found that the coated workpiece does not perform up to required standards. For example, the coated workpieces may not exhibit the desired corrosion resistance or paint adhesion characteristics. This can lead to increased scrap rates and potential recalls, which can drive up costs of manufacturing. Thus, it is desirable to provide a pretreatment coating composition that has a longer pot life, meaning that a metal workpiece can be immersed in the bath for a longer period of time without a decrease in the performance of the coated metal workpiece in corrosion resistance or paint adhesion.
It is also desirable to provide increasing functionality in terms of enhanced corrosion protection and improved paint adhesion in pretreatment coatings to a wide range of metal substrates. At the same time, these improvements preferably do not require changes to existing industrial processes or the equipment used on these processing lines.
Many zirconium-based conversion coating baths contain copper, either as an additive to improve features of the pretreatment coating and/or process or as a trace element from water or metal workpieces being coated. Regardless of its source, the present inventors have discovered that copper from the zirconium-based coating bath that is deposited in the pretreatment coating at too high an amount relative to other coating components can negatively affect performance of the coated metal substrate. Accordingly, it is desirable to develop zirconium-based coating baths that overcome this deficiency.